The present invention relates to a dry toner used to develop electrostatic images in electrophotography, electrostatic recording, electrostatic printing or the like by means of heat fixing.
A dry toner is usually produced by dispersing various agents such as releasing, coloring and charge control agents in a binder resin, grinding the resultant dispersion to toner size by fine grinding means, and classifying the powders into toner particles. Depending on the development system used, the dry toner is broken down into a one-component toner and a two-component toner comprising toner particles and carrier particles.
In recent years, faster operations and lower-temperature fixation than ever before have been required for electrophotography, and binder resins forming toner particles have been required to have lower-temperature meltability, accordingly. For instance, JP-A 63-174061 discloses a dry toner containing as a binder resin a product of reaction between a compound having at least one isocyanate group and a monoalcohol and/or a product of reaction between a compound having at least two hydroxyl groups and a monoisocyanate, and shows that these reaction products are each a low-molecular compound having a low melting point. Although this binder resin has an effect on fixing temperature decreases through its low melting point, yet some problems remain unsolved; decreased offset resistance and limited toner durability may otherwise cause filming with respect to transfer rolls, etc., and storability and blocking resistance become worse.
Likewise, JP-A 63-66564 discloses a dry toner containing as a binder a product of reaction between a compound having at least one hydroxyl or amino group and a monoisocyanate or polyisocyanate compound, and shows that the reaction product is a compound having substantially a single molecular weight of 500 or less and a melting point of 50 to 150xc2x0 C. Although this binder has an effect on fixing temperature decreases through its low melting point for the same reason as mentioned above, yet such problems as mentioned above again remain unsolved.
To meet demands for much faster operations and much lower-temperature fixation, an internal dispersion type of oilless fixing toner particle system with a releasing agent dispersed in a binder resin is now proposed. For instance, JP-A 09-34170 discloses a dry toner comprising a binder resin, a coloring agent and a releasing agent, and teaches that the binder resin comprises an urethane resin as an example and has an average molecular weight of 50,000 or greater and the releasing agent has a specific melting point, so that the toner can be improved in terms of low-temperature fixability, offset resistance and blocking resistance. However, the addition of the releasing agent causes internal cohesive force to become weak, and so care must be taken so as to compensate for offsets such as deposition of toner onto a fixing roller. To shirk this, the content of the releasing agent must be increased. As a result, however, toner durability does not only become worse but it is also difficult to achieve the optimum dispersing conditions for the releasing agent, the optimum grinding conditions, etc. In addition, the incorporation of the releasing agent in the amount than required offers a transparency problem, for instance, in the case of a color toner.
Generally, styrene-acrylic copolymers are used for toner binders. For example, JP-A 06-27731 discloses to carry out polymerization reactions in a mighty mixing machine such as a double-shaft extrusion reactor for the purpose of continuous toner binder production. However, this continuous production has a grave problem because some steps must be provided for removal of an increasing amount of unreacted monomers. Polyester resins used for toner binders as is the case with the styrene-acrylic copolymers, too, has a similar problem in conjunction with their continuous production, because water and other components must be discharged from the system for the purpose of controlling reactions at a polymerization reaction step.
Thus, a primary object of the present invention is to achieve a dry toner that is of improved low-temperature fixability, pulverizability, offset resistance, durability and storability as well as improved transparency and light resistance and so provides a useful dry color toner, and is improved in terms of continuous productivity as well, and provide a dry toner production process.
The dry toner according to the present invention is characterized by comprising as a binder resin a polymer (hereinafter called a binder polymer) that is obtained by bulk polymerization of a compound having at least two isocyanate groups (hereinafter called a polyisocyanate) and a compound having at least two functional groups, each containing active hydrogen (hereinafter called a polyactive hydrogen compound), contains an urethane bond or urea bond in its main chain and has a number-average molecular weight, Mn, of 1,500 to 20,000 as measured on a polystyrene basis.
The dry toner of the present invention is characterized in that the aforesaid compound having at least two isocyanate groups is a polyisocyanate represented by the following formula (1): 
wherein R1 is an alkylene group selected from the group consisting of methylene, ethylene and xe2x80x94C(CH3)2xe2x80x94 groups, and R2 and R3 are each a group selected from the group consisting of an alkyl or alkoxy group having 4 or less carbon atoms and a halogen.
The dry toner of the present invention is characterized in that the aforesaid compound having at least two isocyanate groups is an alicyclic diisocyanate compound wherein two isocyanate groups are attached directly or via an alkylene group to a cyclic aliphatic hydrocarbon.
The dry toner of the present invention is characterized in that the aforesaid alicyclic diisocyanate compound is isophorone diisocyanate.
The dry toner of the present invention is characterized in that the aforesaid alicyclic diisocyanate compound is a polyisocyanate represented by the following formula (2): 
wherein R4 is selected from the group consisting of a single bond, a methylene group, an ethylene group and a xe2x80x94C(CH3)2xe2x80x94 group, l and m are each an integer of 1 to 5, and n is an integer of 0 to 2.
The dry toner of the present invention is characterized in that the aforesaid compound having at least two functional groups, each containing active hydrogen, is a polyoxyalkylene bisphenol A ether compound represented by the following formula (3): 
wherein R and R may be identical with or different from each other and are each an ethylene or propylene group, and x and y are each an integer of 1 or greater with the proviso that the average value of x+y is 2 to 12.
The dry toner of the present invention is characterized in that the aforesaid polymer has a weight-average molecular weight (Mw) to number-average molecular weight (Mn) ratio of 1.5 to 20.
One dry toner production process of the present invention is characterized by obtaining a polymer containing an urethane bond or urea bond in its main chain and having a number-average molecular weight (Mn) of 1,500 to 20,000 as measured on a polystyrene basis by bulk polymerization of a polyisocyanate and a polyactive hydrogen compound in the absence of any solvent, and kneading a coloring agent with the polymer, followed by grinding.
This dry toner production process is characterized in that the aforesaid bulk polymerization is continuously carried out in a belt form of reactor passing through a reaction furnace set at a reaction temperature.
This dry toner production process is characterized in that the aforesaid bulk polymerization is continuously carried out using a double-shaft extrusion reactor.
Another dry toner production process of the present invention is characterized in that:
a polyisocyanate compound and a polyactive hydrogen compound are mixed together in the absence of any solvent,
a mixture is continuously fed into a double-shaft extrusion reactor built up of a barrel, a barrel inlet, a side feeder provided on an intermediate portion of said barrel and a barrel outlet from the barrel inlet for bulk polymerization, thereby obtaining a polymer containing an urethane bond or urea bond in its main chain and having a number-average molecular weight (Mn) of 1,500 to 20,000 as measured on a polystyrene basis, and
a pigment-containing additive is continuously fed from said side feeder, a kneaded mixture of said pigment-containing additive and said bulk polymerization product is continuously discharged from said barrel outlet, and the thus discharged kneaded mixture is ground.
The dry toner of the present invention is of improved low-temperature fixability as well as improved pulverizability, offset resistance, durability, storability, transparency and light resistance, and the production process of the present invention lends itself to continues production.